Fallalings or Mantlings. Are Early ancestors of elves who made a deal with demons to get out of the deal with Aengus. The demons split their indentity into two, and so they were no long true guardians, but half a demonic being and half-fair.
They cannot be detected by true seeing, as both forms are their true forms, and their magic protects them from weak forms of transformation spells cast upon them unwillingly.

Names

Fallalings use elven names, but they also give their mantle a name once they can change into the hybrid demon form.
Mantle Name: Agni, Aldebrand, Aodh, Azar, Bast, Brander, Cinaed, Fintan, Joash, Keahi, Ugne.  

Living

Most Fallalings stay in the feywild, and if not, they live in a city where the veil is so thin that no one will be the wiser. Other elven races consider them cursed and bad luck. Only the half-elfs find a common "pain" to share with them. Dwarfs do not find their nature amusing, actually, they repulse them more than any other of the elven kind. So Fallalings mostly find themselves in fair-touched or tiefling company. Half-lings also don't mind them but they do have many superstitious beliefs on them. Gnomes find them wonderful and wonderous but most fear them too much to even approach them. Any good cleric of any ancestry finds them as an abomination upon the earth. If they even find what they are.  

History

It's not just not public knowledge, not even known for the Fallaling. The Myth is written about only in a few books and only elder elven even knows it exists. For the most part, they made a culture all together far from demons, for the most part, they are fey. It is a bad joke, but most of the books about the fallaling were written by Gnomes or other races who wanted to study them. Fallaling have their mantle to care about, they need to study themselfs they dont have the time to study others.  

Fallaing Traits

Your fallaling character has a variety of natural abilities, the result of the unholy combanation of demons and fey.

Ability Score Increase

Your Charisma score increases by 1.  

Alignment

Fallalings are the embodiment of two slaves in one body, and they must work together to achieve any kind of greatness, so they lean strongly toward the gentler aspects of lawful. They value and protect others’ laws as well as their own, and they are more often good than not. The Bone Fallaing are an exception; their mantle will fight them to keep doing evil deeds.  

Size

Your size is Medium.  

Speed

Your base walking speed is 30 feet.  

Darkvision

Accustomed to twilit forests and the caravans of the nine hells, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of red or blue.  

Two Forms Same Body.

Your nature cannot be detected by means of true seeing, and when you die your body stays in the last form you chose to be in. Changing forms does not change your stats.  

Languages

You can speak, read, and write Common, Infernal and Elvish.  

Subraces

The Bone Fallaling

Those fallalings appear as a husk of elfs. The skin around their face is taken back to the point they are no more than a talking skull with big white holes as eyes. Bone fallaing mantle seems to have some sadism bound into them. They love to inflict pain and they gloat at others sorrow. Decy is a beauty for them, and death a necessity.
The Bone Traits  

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Ability score improvment
Your dexterity score is increased by 1
Bone body
The dreaded mantle of the bone demon bestowed you with a gift. When you change into your hybrid form, wings or a tail with sting sprouts out of your body.
Sadistic Image
When angered or when you really despise someone, you may start having thoughts about how to kill him. As a bouse action, you may project your thoughts to a creature's mind if he is within 60ft of you. Images that are projected in that way give you advantage to intimidation checks against that creature. It must have intelligence score of 4 or higher to affected.  

The Ice Fallaling

Insect-like eyes sport forth on their four heads they walk on bent legs like a mantis and are able to jump to great highest. They believe they are the best of the bunch. Taking after ice demons they should rule over all other fallaling kind.
The Ice Fallaling Traits  

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Ability Score Increase
Your wisdom score increases by 2.
Resistance
You gain resistance to cold damage.
Superior Movment
Your base walking speed is 35 feet.
Outstanding Leap
You have advantage on all Athletics or Acrobatics checks made to jump or climb.

The Horned Traits

 

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Ability Score Increase
Your strength score increases by 2.
Devil's Cut
As a free action, When you hit a creature with piercing or slashing damage you may worsen the wound. If the target is a creature other than an undead or a construct, it must succeed on a DC (10+your constitution score) Constitution saving throw or lose 2 (1d4) hit points at the start of each of its turns due to an infernal wound. Each time you slash or pierce the wounded target with a new attack, the damage dealt by the wound increases by 2(1d4). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing. Targets who healed or saved from the effect are immune to it for 24 hours.
Infernal Rage
As a reaction, when attacked you might unleash your inner demonon the attacker. Your eyes will glow red as they seek the creatures weak spots, you gain a +10 bonus to all perception checks and to passive preception until the next attack you make. If you can still see the creature who attacked you and attack him, you make the attack with advantage. Raging is very taxing, you may only do it once per short rest.